1. Field of the Invention
The present invention relates to a spin valve sensor with a spin filter and specular reflector layer and, more particularly, to a spin valve sensor which has a free layer structure and/or pinned layer structure with such a layer composed of half metallic phase iron oxide (Fe3O4).
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
There is a continuing effort to increase the magnetoresistive coefficient dr/R of the spin valve sensor. As indicated above a greater difference between the resistances of the spin valve sensor between the case where the magnetic moments of the free and pinned layers are parallel and the case where the magnetic moments of the free and pinned layers are antiparallel will result in a greater magnetoresistive coefficient dr/R. It is a purpose of this invention to increase the aforementioned difference of the resistances of the spin valve sensor so as to increase the magnetoresisitve coefficient dr/R.
The present invention provides the spin valve sensor with a half metallic phase iron oxide (Fe3O4) in association with the free layer structure and/or the pinned layer structure. The iron oxide layer serves a dual purpose, namely: (1) it reflects majority electrons back into the spin dependent region of the sensor and (2) it filters out minority electrons so that they are no longer present in the spin dependent region. The classification of electrons as majority and minority electrons depends upon the orientation of the magnetization of the layer (free or pinned) through which the electron is conducted. An example is where the iron oxide interfaces the pinned layer with the pinned layer located between the spacer layer and iron oxide layer. Assuming a first case where the magnetization of the pinned layer is directed upwardly into the sensor and a signal field has rotated the magnetization of the free layer upwardly into the head, electrons which spin downwardly are minority electrons and will be filtered out of the spin dependent region by the iron oxide layer and the electrons which spin upwardly in the same direction as the magnetization of the free layer are majority electrons and will be reflected back into the spin dependent region by the iron oxide layer. This is a low resistance state of the sensor to the sense current. Assuming a second case where the magnetization of the pinned layer is still the same but the magnetization of the free layer has been rotated downwardly out of the head, the spin down electrons are still filtered out of the spin dependent region by the pinned layer structure since they are antiparallel to the magnetization of the pinned layer, but the spin up electrons are now antiparallel to the magnetization of the free layer structure and are minority electrons and have a short mean free path which raises the resistance of the sensor to the sense current. As compared to a spin valve sensor without the iron oxide layer the difference between the low and high resistance state of the sensor is greater in the spin valve sensor with the iron oxide layer which results in a greater magnetoresistive coefficient dr/R. Additional information on the conduction electrons can be found in commonly assigned U.S. Pat. No. 5,422,571 which is incorporated by reference herein.
Another important advantage of the iron oxide layer is that it performs its improvement of the magnetoresistive coefficient dr/R without shunting the sense current. This is because the iron oxide layer is an insulator instead of a conductor. It should be understood that the output signal of the sensor is reduced when the sense current is shunted by a conductive layer in the sensor.
An object of the present invention is to increase the magnetoresistive coefficient dr/R of a spin valve sensor without shunting the sense current.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.